This invention relates to a top-and-bottom blown converter of the type wherein refining gases are blown into molten metal through a top-blowing lance inserted through a converter top opening and a plurality of tuyeres arranged at the bottom to refine the molten metal, and more particularly, to the position of a sub-lance directly immersed into the molten metal for measuring the temperature, carbon concentration and other factors of the molten metal during blowing.
As is well known in the steel making art, the blowing operation of a converter is conducted under controlled blowing conditions so that the desired carbon concentration and temperature of molten metal may be reached at the time of blowing-out. To this end, it has been a general practice to insert a sub-lance through a converter top opening during blowing to directly measure the carbon concentration and temperature of the molten metal by means of a probe at the lower tip of the sub-lance.
As a substitute for the conventional oxygen top-blown converter or LD converter, great attention has recently been paid to a bottom-blown converter of the type wherein refining gas is blown through tuyeres at the bottom. This process is also known as the quality basic oxygen process (Q-BOP). The bottom-blown converters have some advantages over the LD converters in that molten metal is more vigorously agitated with the gas blown through the bottom tuyeres to further reduce the concentration of iron value in the slag or the total iron in the slag (abbreviated as T.Fe), resulting in an improved iron yield. It is, however, expensive to replace the existing LD converters by bottom-blown converters. A compromise is to modify an existing LD converter by adding bottom blown tuyeres at the bottom thereof. Such a modified converter provides the advantage of bottom blowing without the sacrifice of the existing LD converter installation.
In the modified top-and-bottom blown converter, molten metal is vigorously agitated by the gas blown through the bottom tuyeres as in the genuine bottom-blown converter, causing the molten metal to vigorously wave or vibrate. When a sub-lance probe is inserted into such vigorously vibrating molten metal for the purpose of measuring the carbon concentration and temperature of the molten metal as described above, there is the likelihood that the probe will be broken or damaged to make measurement impossible. Particularly when gas is bottom blown at an increased flow rate, not only the probe, but also the sub-lance itself are broken or bent. The risk of steam explosion would result from ejection of coolng water circulating in the sub-lance and it becomes difficult to move the sub-lance vertically up and down. To overcome these problems, the inventors have made investigations of sub-lance failure to reach the following findings.
A group of bottom blowing tuyeres are often arranged in a top-and-bottom blown converter at intervals in alignment with a line which extends across the center of the converter bottom and runs parallel with a trunnion axis. The molten metal is waved such that opposite portions of the molten metal divided with respect to the line of alignment of the tuyeres will alternately move up and down as if mercury in a U-shaped tube moves alternately oppositely in a longitudinal direction of the tube under gravity. The wave becomes more vigorous as the distance from the line of alignment of the tuyeres becomes greater. Since commercially available top-and-bottom blown converters are obtained by modifying existing LD converters, the top lance for blowing a refining gas is usually designed so as to move in alignment with the vertical axis of the converter as in the case of LD converters. Thus, the monitoring sub-lance is generally moved vertically at a position spaced a distance of 1 m or more from the vertical axis of the converter. The monitoring sub-lance is immersed in a portion of the molten metal where waves are most severe. Consequently, the sub-lance undergoes severe impact from the molten metal and often experiences an accident of breakage or failure.
It is thus an object of this invention to minimize the impact force applied to a sub-lance probe and sleeve by molten metal, thereby preventing any accident of failure or bending of the sub-lance probe and sleeve.